1. Field of the Invention
The present invention relates to a method of converting a polyamic acid to a polyimide by treatment with microwave energy. More particularly, the invention relates to the conversion of polyamic acid to a non-foamed polyimide in the absence of a solvent by subjecting the polyamic acid to focused microwave radiation.
2. Description of the Background
Polyimides are an important class of high-performance polymers which have applications as composites in the aerospace industry and as high temperature coatings in the electronics industry. A problem with polyimide preparation is the difficulty of processing the precursor polyamic acids to guarantee complete conversion to the corresponding polyimides. This problem has limited the widespread use of these materials. Factors which are known to be responsible for these processing problems include poor solubility in common solvents, evolution of volatiles during heat treatment and void formation. (K. L. Mitta, "Polyimides, Syntheses, Characterizations, and Applications", Plenum, New York, 1984, volumes I and II; M. I. Besonov, M. M. Koton, V. V. Kudryavtsev and L. A. Laius, "Polyimides, Thermally Stable Polymers", Consultants Bureau, New York, 1987). A most significant problem, however, is the long time and high temperatures which are required to obtain complete conversion of the polyamic acid to polyimide, according to the reaction scheme shown in FIG. 1. (A. K. St. Clair and T. L. St. Clair, ACS Symposium Series No. 346, M. J. Bowden and S. R. Turner, ed, p. 437 (1987)).
Several studies in the literature (J. A. Krevz, A. O. Endrey, F. P. Gay and C. E. Sroog,, J. Polym. Sci., Part A-1 (1966), 2067; S. V. Lavrov, I. E. Kardash and A. N. Pravednikov, Polym. Sci., USSR (1977), 19, 2727; S. V. Lavrov, A. Y. Ardashnikov, I. E. Kardash and A. N. Pravednikov, Polym. Sci., USSR (1977), 19, 1212; E. Pyun, R. J. Mathisen and C. S. P. Sung, Macromolecules (1989), 22, 1174) have attempted to define the essential features of the imidization mechanisms and kinetics. However, these previous studies, especially the solution studies, have often been either contradictory or incomplete. This has often led to an adoption of complex, empirically established, multi-stage processing schemes with a limited amount of scientific understanding.
An area of technology which is known is the preparation of foamed and fully cured polyimide resins by the application of microwave energy. Gagliani et al, U.S. Pat. No. 4,599,365, describes the preparation of foamable polyimide resins by esterifying a dianhydride, reacting the product with a diamine, drying the resulting product and heating this product to spontaneously form a foam. Complete foaming and curing of the polyimide is achieved by incorporating an oxoimine in the polyamic acid. Lanier et at, U.S. Pat. No. 4,855,231, shows the preparation of a polyimide foam by raising the temperature of at least the lower portion of a body of polyimide precursor to a temperature within the range of 50.degree. to about 200.degree. C. and commencing the exposure of the body of the polyimide precursor to microwave radiation of an intensity sufficient to cause development of a foamed polymer structure. The body of the polyimide precursor and of the foamed structure, as it develops therefrom, is maintained under a substantially vapor-impermeable microwave compatible shroud that does not impede the development of the foamed structure. Broemmelsiek et al, U.S. Pat. Nos. 4,877,563 and 4,900,762, show a method of preparing a foamed polyimide by subjecting a polyimide precursor to microwave radiation directed to the body of the precursor from the top, sides and ends thereof. The method avoids curtailment of radiation such that it is not directed upwardly into the precursor or developing foam. A preferred method of curtailment involves maintaining a microwave radiation curtailing shield between the precursor body and the source of microwave radiation therebelow. Wright, U.S. Pat. No. 4,892,896, discloses a process for transforming a normally non-flowable resinous semi-solid or viscous liquid polyimide precursor into a flowable liquid. The precursor composition is subjected to microwave radiation of an intensity sufficient to convert at least a portion of this composition into a flowable liquid without causing a significant change in the chemical composition of the precursor composition. Finally, Lee et al, U.S. Pat. No. 4,897,432, discloses a method of improving the yields of polyimide foam by keeping a body of a polyimide precursor under a substantially vapor-impermeable microwave-compatible shroud, before and during the time the body of polyimide precursor is exposed to microwave radiation, so that development of a foamed structure is not substantially restricted or impeded. These methods of the prior art are essentially directed to the preparation of polyimide foams in a microwave oven. Further, the microwave power levels employed are much higher than the 40 W maximum level of the present process.
Recently, microwaves as a heating source for the imidization reaction has been suggested for decreasing the reaction time of the polyamic acids. (D. A. Lewis, J. D. Summers, T. C. Ward and J. E. McGrath, Accelerated Imidization Reactions Using Microwave Radiation, Polymer Preprints, 29(1), 174 (1988)). This work is based upon the reaction between 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA) and 3,3'-diaminodiphenylsulfone (3,3' DDS) in solution. Here the reaction times are reduced by a factor of 20 to 40 when using microwaves, depending upon the reaction temperature. No mention of performing the reaction in the solid state is disclosed in the publication. In similar work by L. A. Fellows and M. C. Hawley of Michigan State University, a polyamic acid (LARC-TPI) formed from 3,3'-DDS and BTDA is subjected to microwave radiation in a diglyme solution. No mention is given of reduced reaction times and, of course, the reaction is conducted in solution and not in the solid phase.
An interesting result from this work is the determination that the activation energy of the microwave-induced reaction was about 57 kJ/mole and of the thermal process 105 kJ/mole. It has been proposed that the observed decrease of activation energy is due to the fact that the reacting groups in the reaction are "hotter" than the bulk, because of higher "local" vibrational and rotational energy levels in the reacting molecules in comparison with neighboring groups. The temperature difference due to preferential absorption of the microwaves by the acid and amine groups is evaluated to be around 50-60.degree. K. However, no direction is given in said publication, about a process for obtaining a solid polyimide under treatment with microwaves which is free of voids, homogeneous and endowed with a satisfying combination of mechanical, thermal and electrical properties.
A need continues to exist for the development of a polyamic acid to polyimide technique which is effective for achieving a solid non-foamed plastic material and which can be accomplished by the application of microwave energy.